When designing a low-noise amplifier (LNA) it is important to consider its required input matching (e.g. to 50Ω), matching bandwidth, noise figure, linearity and power consumption. If the LNA has a well behaved input impedance the matching network will be easy to design and robust in production. If the resistive component of the LNA's input impedance is very far from the desired matching impedance (e.g. 50Ω) it is very difficult to match it properly without adding extra resistive losses, and, hence, noise. Also a wide-band matching network will be more complex than a more narrow-band one. To keep cost and size down it is important that an LNA can be matched to several input frequencies. This requires wide band LNA structures. Finally, it is normally desirable to have a very high LNA input compression point without sacrificing power consumption.
Two common methods are used for setting the resistive part of the LNA input impedance: resistive shunt or inductive series degeneration. The inductive series feedback structure provides good noise properties but is inherently narrow band and is, thus, not suitable for wide-band matching or multi-band applications and will, typically, require an external matching component or network. The resistive shunt feedback has the advantage of providing an integrated wide-band input match but suffers from noise figure degradation due to the resistive feedback element and due to the fact that some of the drain/collector current, and its noise component, is fed back to the input via the shunt element.
To circumvent some of these shunt feedback noise issues an active wide-band input matching technique, in the following referred to as “resistive shunt feedback noise canceling”, has been proposed by Bruccoleri et al (F. Bruccoleri, E. A. M. Klumperink, and B. Nauta, “Noise cancelling in Wideband CMOS LNAs,” in IEEE International Solid-State Circuits Conference Digest of Technical Papers, 2002). This technique, illustrated in FIG. 1, exploits the fact that the drain noise of the impedance setting device, e.g. M1 in FIG. 1, is correlated and in-phase at the drain and gate terminals while the signal is anti-phase which leads to a cancellation opportunity.